The present invention relates to the inhibition of the corrosion of metals by vapors to which the metals are exposed. More particularly, the invention relates to the inhibition of the corrosion of electrical components disposed in an enclosure.
It is well known that the presence of water molecules adsorbed on surfaces as a result of exposure to humid atmospheres enhances metal corrosion in such an atmosphere. It is likewise known that water molecules adsorbed on the surface of electrically-insulating materials can promote sufficient undesired electrical conduction between circuit components as to severely disrupt high-impedance electrical circuits. Airborne contaminants such as hydrogen sulfide, chlorine, and salt particles, particularly under conditions of high humidity, are a major cause of electrical circuit component corrosion and subsequent failure.
As used herein, the term "impedance" is meant to signify any form of electrical resistance, either to direct current or to alternating current.
The concept of employing dessicants and vapor-corrosion inhibitors independently and physically separated within the same enclosure is known in the art. A problem frequently encountered is the need for frequent and/or periodic replacement of the dessicant. Typical service life for a dessicant packet is from about three to six months. This situation and condition often precludes the use of dessicants in combination with but physically separated from vapor-corrosion inhibitors.
Vapor-corrosion inhibitors are materials which inhibit corrosion of the surface of metals contacted by vapors of the corrosion inhibitors. Ideally, a vapor-corrosion inhibitor would vaporize at a rapid rate when first placed in service, to provide immediate protection to electrical components within an enclosure, and thereafter vaporize at a slower rate, to increase the duration of protection. Many patents and scientific articles teach methods which attempt to achieve this ideal condition. Most prior-art techniques employ either mixtures of inhibitors having a wide range of vapor pressure, or provide means for limiting the vaporization and/or vapor diffusion rate of the inhibitors. Both approaches limit the choice of the vapor-corrosion inhibitor which can be utilized.
U.S. Pat. No. 2,577,219 to Wachter et al, issued Dec. 4, 1951, discloses a method of preventing or inhibiting corrosion of metal surfaces by employing a plurality of vapor-corrosion inhibitors in the presence of each other under conditions in which at least two of the inhibitors are complementary to one another.
U.S. Pat. No. 2,643,176 to Wachter et al., issued Jun. 23, 1953, discloses compositions for protection of metals against corrosion. The compositions comprise a substantially solid material which contains, or is impregnated or coated with, a vapor-corrosion inhibitor.
U.S. Pat. No. 2,752,221 to Wachter et al, issued Jun. 26, 1956, discloses methods and compositions for use in protecting metals from corrosion, especially by water vapor and oxygen, as in humid air. The vapor-corrosion inhibitors comprise a basic agent and a water-soluble organic nitrite.
U.S. Pat. No. 3,836,077 to Skildum, issued Sep. 17, 1974, discloses a device for protecting structures from corrosion during storage. The device includes a carrier defining at least one opening therein. The opening contains a mixture of organic ammonium nitrites with varying vapor pressures, a chemical buffer system for neutralizing lead acids, and a volatile anti-oxidant for preventing the formation of varnish and scavenging oxygen from varnish deposits.
U.S. Pat. No. 3,967,926 to Rozenfeld et al., issued Jul. 6, 1976, discloses a method for inhibiting atmospheric corrosion of metals in a sealed space with inhibiting amounts of vapor-phase inhibitors. The method consists of disposing in the sealed space a carrier for storing a stock of inhibitors, and diffusing their vapors within the space. The carrier is silica gel or zeolite, and contains a liquid inhibitor selected from the group consisting of primary, secondary and tertiary amines, and mixtures thereof.
U.S. Pat. No. 4,275,835 to Miksic et al., issued Jun. 30, 1981, discloses a corrosion-inhibiting device which includes an extremely stable, man-made synthetic carrier having chemical and physical stabilities compatible with hostile and adverse environments, for dispensing corrosion-inhibiting chemicals.
Scientific or technical articles which review the role of vapor-corrosion inhibitors in the electronics industry include "Corrosion Inhibitors in the Electronics Industry: Organic Copper Corrosion Inhibitors," by D. Vanderpool, S. Akin and P. Hassett, Corrosion/86, Paper No. 1, Houston, Tex., 1986; "COBRATEC.RTM. Inhibitors: Corrosion Protection for Electronics," by Gilbert K. Meloy, PMC Specialties Group, Inc., Cincinnati, Ohio; and "Volatile Corrosion Inhibitors for Protection of Electronics," by Michael E. Tarvin and Boris A. Miksic, Corrosion/89, Paper No. 344, Apr. 17-21, 1989, New Orleans Convention Center, New Orleans, La.